A conventional data packet frame or data packet is made up of a header and an information frame which are transmitted in sequence typically at identical communication rates. The header contains, among other things address information which is used to route the data packet through a series of switching nodes to its correct destination. At each node the packet is stored in memory and the target or final destination address is determined from the header. The switching node decodes the header, and switches the packet onto the proper communication link so as to proceed to its proper destination within the network.
In current fiber optic data packet networks switching of an optical data packet utilizes electrical rather than optical processing of the header. This in turn requires complete signal regeneration to be performed at each switching node at baseband. Tapping off a small portion of the optical signal and using this signal to extract the header information is precluded because the header and the information frame have the same data communication rate, and therefore the same optical power is required to detect the header as the information frame.
In an alternative approach, the header is transmitted at a slower rate as described in Ha, et al. "Demonstration of Photonic Fast Packet Switching at 700 Mb/s Data Rate", Electronics Letters, Vol. 27, pages 789-790 (1991). In this approach, a small portion of the optical signal can be tapped and used to process this optical header information. Since the optical signal power required to satisfy a predetermined maximum bit error rate increases non-linearly with an increasing data rate, less optical power is required to detect a slow header. Further, the integrated circuits that decode the header can operate at slower rates than the baseband information data rate and thus are less complex and less expensive to fabricate. However, there are disadvantages with this approach. This transmission format is incompatible with existing and emerging optical network standards such as SONET and FDDI. Furthermore, the existence of two data rates makes it difficult to optimize the detection bandwidth for the header and for the information data, and clock recovery is more complex. Moreover, since the header is transmitted at a slower rate, the overall throughput for the data packet is decreased, and the data transfer efficiency is lower.